This invention relates to Raman spectroscopy. More specifically, this invention relates to use of a laser enhanced ionization detector connection with Raman spectroscopy.
The detection of one spectroscopic process by observing its affect upon another spectroscopic process is termed resonance detection. Such detectors have been resonance monochromators, especially used to detect atomic fluorescence. The use of resonance monochromators for atomic fluorescence has been based primarily upon the generation and detection of fluorescence in a separate cell. However, Matveev has proposed in the Journal of Analytic Chemistry U.S.S.R., 38,561 (1983) an ionization detector for laser excited atomic fluorescence.
Raman spectroscopy involves the application of an intense beam of monochromatic radiant energy to a sample gas, liquid, or solid. Under these circumstances, a small fraction of the energy is scattered. In addition to the original frequency of the applied radiation, the scattered radiation contains a small fraction at a lower frequency and an even smaller fraction of radiation at a higher frequency. The differences between the frequencies of applied radiation and the scattered rays correspond to vibration and rotation frequencies of the irradiated molecules. Thus, Raman spectroscopy may be used to reveal the fundamental frequencies of molecules from measurements. The measurements are usually in the visible and ultraviolet part of the electromagnetic spectrum and Raman spectroscopy often supplements infrared spectroscopy.
A known approach to Raman spectroscopy involves inducing the Raman scatter by a laser, collecting the scatter by suitable optics, transferring the Raman scatter to a double or triple monochromator, and detecting by use of a photomultiplier tube. Spectra are obtained by scanning the monochromator over a wavelength region near to the wavelength of the laser which is producing the Raman scatter. Although Raman spectroscopy has been quite useful, there are problems in its application. In particular, the intensity of scattered light and the efficiency of detection are very low. Accordingly, the sensitivity of the technique is poor, usually limited to concentrations of 0.1% and greater. In addition, spectra may not be obtained, except with great difficulty, at wavelengths close to the exciting laser because of stray light produced in the monochromator by Rayleigh scattering from the sample. The Rayleigh scatter from a fixed wavelength excitation laser makes it necessary to use a large double spectrometer with poor optical efficiency.
A more recent approach to Raman spectroscopy involves laser induced scatter occurring in the near infrared region. The scatter is collected and transferred by suitable optics (including a source line rejection filter) to a Michelson interferometer and detected by a photodiode. The produced interferogram is treated by Fourier transformation to give the Raman spectrum. Although this approach is useful to a certain extent, the rejection of stray laser light scatter again causes a loss of signal (and thus lowers signal-to-noise ratio). This results in poor detection limits even when resonance enhancement and/or surface enhancement techniques are used. The conventional and interferometer approaches also are characterized by restricted spectral ranges which are limited at the low energy end by the laser profile, laser intensity, and spectral rejection of the system, and by a nominal resolution which is often limited by the need for reasonable spectral band pass in the dispersive system or short mirror near travel in the interferometer.
A spectroscopic scheme for lithium is shown in FIG. 1 and will be used to explain the operation of a known laser enhanced ionization device of FIG. 2. Two strong bound-bound transitions are used, one, at 670.78 nm originating from the ground state and terminating at a level of 14904 cm.sup.-1 and the other, at 460.29 nm beginning at that level and terminating at 36623 cm.sup.-1, 0.85 eV below the ionization limit. If two optically saturating laser beams enter coincidentally into a lithium atomic vapor (eg., an air/H.sub.2 flame into which a lithium solution has been introduced) at these wavelengths, the promotion of lithium to the ionic state can be made to be 100% efficient due to the efficiency with which excited species in the 36623 cm.sup.-1 level are collisionally ionized. Moreover, the collection of these charged species can be 100% efficient within the electric field provided by the immersed electrode. Such an experiment is the basis for the technique of Laser Enhanced Ionization (LEI) which has been used successfully for the sensitive detection of many atomic species.
With reference now to FIG. 2, the electrode is supplied with a negative high voltage and the base of the burner serves as the other electrode. Lasers of two frequencies enter the space between the burner and the electrode. One of the frequencies corresponds to the transition from the ground state to the first excited state in FIG. 1 whereas the other of the laser wavelengths corresponds to the transition from the first excited state to the second or higher excited state. The flame from the burner is used to provide the energy needed to ionize the sample gas which is supplied to the burner.
The device of FIG. 2 is a true resonance device in that the spectroscopic transition being detected is the same as the transition within the resonance monochromator. The device or system of FIG. 2 is useful for detecting atomic species, but is not applicable to the detection of Raman scatter.